A generally and widely employed destructive measurement method for measuring hydrogen concentration in a metal material is, as shown in ASTM E1447-92, a method wherein: an object to be measured is cut into fragments of 1 gram or less, dried after washing, transferred into a heat-resisting crucible, melted at the melting point of the sample or a lower temperature with a flux composed of pure metal; hydrogen is extracted together with other gases from the object to be measured; the amount of the hydrogen gas is determined by the change in the thermal conductivity of the extracted gasses collected in a fixed container; and the hydrogen concentration is calculated.
However, the implementation of the above method is often practically difficult because the object to be measured is subject to destruction. Particularly, if the object is a radioactive metal, the problem of excessively large cost and facility required for disposal of the contaminants arises.
For these reasons, a method for nondestructively measuring hydrogen concentration in a metal material is highly demanded.
As a nondestructive method, firstly there is a method making use of electromagnetic properties.
In this method, an eddy-current sensor probe including a coil is placed on the surface of an object to be measured, and the above coil is excited by applying an alternating current. This allows eddy current generation in the electrically conducting object to be measured by electromagnetic induction. It is known that electromagnetic characteristics of a metal material, including the electric conductivity, magnetic permeability, etc. of the metal, change, though only slightly, depending on the magnitude of the ratio of the hydride precipitates in the metal to be analyzed.
Making use of this, the hydrogen concentration in an object to be analyzed can be detected by measuring the electric conductivity, magnetic permeability, etc. of the object to be measured. Examples of such measurement of the hydrogen concentration in a member composed of titanium alloy, zirconium alloy, etc. using this principle are disclosed in the Japanese Patent Application Laid-open Publication No. 2001-141698 and the official gazette of Japanese Patent Application Publication No. 10-206934/1998.
Another known nondestructive method for hydrogen concentration measurement in the next place is a method by means of an ultrasonic method using a piezoelectric element.
For example, the Journal of Nuclear Material Vol. 252 (1998) discloses on pages 43 to 54 that by determining the longitudinal-wave sound speed of Zircaloy-2 with a plate thickness of 3 mm using a piezoelectric element exclusively for longitudinal-wave generation at around 10 MHz, and then determining the transverse-wave sound speed at the same location using an exchanged piezoelectric element exclusively for transverse-wave at around 2 MHz, the hydrogen concentration can be estimated from the ratio of the aforementioned sound speeds provided that a specific relation exists between the ratio of the sound speed of thus-obtained transverse wave to that of thus-obtained longitudinal wave and the hydrogen concentration.
As yet another nondestructive method for hydrogen concentration measurement, a method combining the phenomena of electromagnetic induction and ultrasonic resonance (an electromagnetic ultrasonic resonance method) has been proposed.
With regard to this method, the official gazette of the Japanese Patent Application No. 2000-375957 (the Japanese Patent Application Publication No. 2002-181795) and the Annual Meeting of the Atomic Energy Society of Japan (Spring 2001; Page L-44) disclose an instrument and a method for hydrogen concentration measurement wherein: a permanent magnet or electric magnet and a coil are placed on the surface of an object to be measured; the magnetic flux density in the proximity of the surface of the object is shifted by applying a high frequency wave to the coil by which elastic waves are generated directly on the surface of the object; two transverse ultrasonic waves are received, the vibrations of which deflecting in the directions parallel to the plane of the object and perpendicular to each other; the frequency of the coil is changed so that said two transverse ultrasonic waves form a resonant state; the resonance frequencies (ft, fr) of the two transverse ultrasonic waves are obtained; the relative difference Δf (acoustic anisotropy) between ft and fr is calculated; and the hydrogen concentration in the object t is calculated making use of the relation:Δf=(ft−fr)/f provided that f=(ft+fr)/2and H is provided as the hydrogen concentration of the object,H=a·Δf+b wherein a and b are coefficients. In addition, as a modification of the above method combining the phenomena of electromagnetic induction and ultrasonic resonance (an electromagnetic ultrasonic resonance method), the official gazette of the Japanese Patent Application No. 14-261406/2002 (the Japanese Patent Application Publication No. 2004-101281) proposes a nondestructive method for hydrogen concentration measurement wherein: the magnetic flux density with respect to an object to be measured is changed; using an electromagnetic ultrasonic sensor for receiving the ultrasonic wave generated in the object as a result of such change fixed at such an angle not to allow the long-axis direction of the coil of said electromagnetic ultrasonic sensor and the rolling direction of the object to overlap with each other, two different resonance frequencies corresponding to two kinds of elastic waves deflecting in the rolling direction of the object and the perpendicular direction relative to the aforementioned rolling direction, respectively, are obtained without rotating the electromagnetic ultrasonic sensor; and, based on this, the hydrogen concentration is calculated with high accuracy.
Since the electric conductivity, magnetic permeability, etc. to be measured are significantly affected by the gap between the sensor and the object to be measured in the above conventional nondestructive method for hydrogen concentration measurement making use of the electromagnetic properties, there are rather many technical problems to be solved such as in the calibration method for the distance of the gap between the sensor and the object to be measured.
Also the above conventional nondestructive method for hydrogen concentration measurement by means of the ultrasonic method using a piezoelectric element requires a coupling fluid for making close acoustic contact of the piezoelectric element with an object to be measured, polishing steps for obtaining a smooth surface, etc., thus making the labor complicated. In order to figure out the sound speed ratio of the longitudinal and transverse waves, this method requires either highly accurate measurement of the wall thickness of the object to be measured at the intended location or resonance frequency measurement with the two waves, longitudinal and transverse waves, at exactly the same location. However, such an object to be measured that industries are generally provided has a surface covered with scales, oxide films, contaminants, etc. and practically it is significantly difficult to determine the wall thickness with high accuracy by means of mechanical procedures. It is also difficult in general industrial situations to alternately contact the sensors, one exclusively for longitudinal waves and another exclusively for transverse waves, with exactly the same location. For these reasons, it has been difficult to yield correct results in the above conventional method using a piezoelectric element. Particularly, assuming application in high-temperature, radioactive or remote environment or to a narrow part of a heat exchanger tube etc. for example, on-site implementation of the sensor exchange is significantly difficult. In view of these facts, the aforementioned procedure using a piezoelectric element is difficult to apply in industry fields and limited to measuring procedures for laboratory studies. The above conventional nondestructive methods for hydrogen concentration measurement combining the phenomena of electromagnetic induction and ultrasonic resonance use the relation as described above: provided that two resonance frequencies of a first transverse ultrasonic wave whose vibration deflecting in the longitudinal (rolling) direction of an object to be measured and a second transverse ultrasonic wave deflecting in the direction at an angle of 90° with respect to said longitudinal direction (a lateral direction with respect to the rolling direction) are fr and ft, respectively, and H is the hydrogen concentration of the object to be measured,H=a·Δf+b                 wherein a, b: coefficientsΔf=(ft−fr)/f Δf=(ft+fr)/2.        
Study data demonstrated that, while the above coefficient a has a substantially constant value in the objects to be measured that are manufactured under the same member specifications (size, components, manufacturing method, etc.), the coefficient b varies within a significant range depending on the location within the object even if the members are manufactured under the same specifications.
That is, in obtaining the acoustic anisotropy Δf of an object to be measured, although a is a constant value by the member specifications, b is unknown because the value of b depends on the object and the location within the object, and thus the hydrogen concentration H cannot be identified.
In other words, if the change of Δf before and after hydrogen absorption in a certain object to be measured is obtained, it is possible to obtain the amount of the hydrogen concentration change by a·(change of Δt), but the absolute value of the hydrogen concentration cannot be calculated with b unknown.
Consequently, in the method proposed in the Japanese Patent Application No. 2000-375957 (the Japanese Patent Application Publication No. 2002-181795), the acoustic anisotropy Δf of a material whose hydrogen concentration is known needs to be figured out in order to obtain the absolute value (H) of hydrogen concentration from Δf. For example, Δf of an object to be measured without hydrogen added (initial state) needs to be measured.
That is to say, the method proposed in the Japanese Patent Application No. 2000-375957 (the Japanese Patent Application Publication No. 2002-181795) has a limit that its application is restricted to a so-called fixed-point observational measurement wherein the acoustic anisotropy is measured at each specific location at the manufacturing stage in advance and then the absolute value of hydrogen concentration is determined by repeating the measurement of the same location.
However, such a nondestructive method for hydrogen concentration measurement that can be applied only to a measurement in a manner of fixed-point observation is inconvenient without being capable of measuring the hydrogen concentration of any given region of an object to be measured. In this connection, the present invention provides the art of obtaining the hydrogen concentration of any given location of an object to be measured at the time of measurement by eliminating the need for the value measured in the state of no hydrogen content to obtain the absolute value of hydrogen concentration, unlike the conventional nondestructive methods for hydrogen concentration measurement by means of the electromagnetic ultrasonic resonance method, to solve the problems.